Automatic ringdown circuit

A variant to manual service is the automatic ringdown circuit or ringdown.

A two-point automatic ringdown circuit has a telephone at each end. When the telephone at one end goes off-hook, the phone at the other end instantly rings.

In some circumstances, the electronics that operate ringing are part of a key service unit. This is the electronics box that operates a multi-line key telephone system. In the wirespring vintage Bell System 1A key service units, a model 216 automatic ringdown was used to operate the circuit.

In other situations, the ringdown is powered and operated by equipment inside the central office.

In the case of enterprises with a PBX switch, the ringdown can be operated by a PBX switch. The switch is programmed to ring a specific extension (the called phone) when a defined extension ( the calling phone) goes off-hook. The PBX does not offer dialtone to the calling extension: it only looks for on-hook or off-hook status.

These circuits are used:

• over high-volume routes where one site calls another very frequently.

Example: an information desk and the information desk staff supervisor's desk.

• where a tamper-proof ability to call from one point to another is needed.

Example: a phone used to summon a taxicab to an airport or hotel.

• where the public or users that are not trained in using a specific office telephone system must place calls.

Example: the after-hours phone to reach the watchman from the front door at a warehouse.

• in locations where emergencies are handled and the time required to dial digits would cause an unacceptable delay in handling of an emergency.

Example: an airport control tower to the airport's fire station or fire dispatch center.

In some cases, automatic ringdown circuits have one-to-many configurations. When one phone goes off-hook, a group of phones is made to ring simultaneously.